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Supplemental Information about Oil & Gas Producing Activities (Unaudited) (Reserve Quantities) (Details)
12 Months Ended
Dec. 31, 2012
MBoe
Dec. 31, 2011
MBoe
Dec. 31, 2010
MBoe
Dec. 31, 2009
MBoe
Reserve Quantities [Line Items]        
Estimates of proved reserves and related valuations based on independent petroleum engineers 100.00%      
Proved Developed Reserves (BOE) 150,216 144,783 133,678  
Proved Undeveloped Reserve (BOE) 124,913 130,143 137,535  
Proved Developed and Undeveloped Reserves, Net BOE 275,129 274,926 271,213 235,303
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE (22,162) (28,446) (3,522)  
Proved Developed and Undeveloped Reserves, Improved Recovery BOE 0 0 1,700  
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions BOE 25,148 30,600 20,019  
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place BOE (556) 0 0  
Proved Developed and Undeveloped Reserves, Production BOE (13,321) (13,025) (11,923)  
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place BOE 11,094 14,584 29,636  
East Texas
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE   (19,632) (3,666)  
Piceance
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE   (5,779) (3,890)  
Uinta
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE     (840)  
Permian
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE     971  
California
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE     3,903  
Utah
       
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE   (2,576)    
Oil
       
Reserve Quantities [Line Items]        
Proved Developed Reserves (BOE) 118,937 107,849 88,917  
Proved Undeveloped Reserve (BOE) 85,271 78,031 77,264  
Proved Developed and Undeveloped Reserves, Net BOE 204,208 185,880 166,181 129,940
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates BOE 12,145 (4,054) 4,288  
Proved Developed and Undeveloped Reserves, Improved Recovery BOE 0 0 1,700  
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions BOE 8,459 19,601 12,774  
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place BOE (556) 0 0  
Proved Developed and Undeveloped Reserves, Production BOE (10,024) (9,041) (7,925)  
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place BOE 8,304 13,193 25,404  
Natural Gas
       
Reserve Quantities [Line Items]        
Proved Developed Reserves (Volume) 187,668 221,606 268,566  
Proved Undeveloped Reserve (Volume) 237,851 312,673 361,626  
Proved Developed and Undeveloped Reserves, Net 425,519 534,279 630,192  
Proved Developed and Undeveloped Reserves [Roll Forward]        
Proved developed and undeveloped reserves, Beginning of year 534,279 630,192 632,178  
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates (205,845) (146,349) (46,860)  
Improved recovery 0 0 0  
Extensions and discoveries 100,129 65,992 43,469  
Property sales 0 0 0  
Production (19,784) (23,907) (23,989)  
Purchase of reserves in place 16,740 8,351 25,394  
Proved developed and undeveloped reserves, End of year 425,519 534,279 630,192